EMI is generally defined to include all potential sources of interference (i.e. unwanted disturbances, or noise) that can compromise the performance of electrical circuits due to electromagnetic coupling including RF interference. EMI is known to propagate in the solid state through conduction over signal and power lines and through the substrate, and also over the air as electromagnetic radiation in free space. For example, RF receivers are known to need good noise immunity for reception of low-level signals.
RF shielding is known for reducing EMI. Traditional RF shielding is implemented with external metal “cans” that encapsulate and shield the RF IC section on a circuit board (e.g., PCB). However, this implementation is costly and time consuming, as the metal cans used for RF shielding must be customized according to individual circuit boards. Additionally, metal cans being external shields increase the space requirements for RF sections and, more significantly, can degrade the performance of the underlying RF circuitry, such as by itself acting as a broadcast antenna of RF noise. This performance degradation generally leads to a time-consuming retuning process that may be necessary to limit the effects of the metal can. Moreover, although metal cans can significantly reduce radiated EMI including radiated RFI, metal cans do not provide protection from EMI coupled through a common monolithic IC substrate (e.g., bulk silicon) referred to herein as “substrate coupling,” as described below.
Another RF shielding arrangement is generally referred to as package-level plating. The plating is applied at the package level (as opposed to wafer level) and the plating is positioned both above and below the RF IC, such as on different layers of a multi-layer PCB. As with metal cans, a limitation of package-level plating is the inability to provide protection from substrate coupling.
Substrate coupling is another form of EMI which involves substrate noise which can couple through a common monolithic substrate (e.g., bulk silicon). Substrate coupling can be a significant problem particularly for monolithic mixed signal RF comprising ICs. In the digital portion of such ICs there are generally a large number of logic gates which generally undergo transitions at a high frequency during normal operation. When such a transition occurs, a spike of current can be absorbed from the power bus. Usually a significant portion of this current is passed through the ground bus through direct feedthrough and a portion is also generally injected into the common monolithic substrate.
Generally, in mixed signal systems, such substrate noise can corrupt the sensitive low level analog circuitry (e.g., RF subcircuits) and thus impair the performance of the mixed signal IC. Substrate noise can generally be reduced by the circuit layout (e.g., larger spacings between digital logic and RF circuitry) and certain isolation techniques. Although substrate coupling can be partially isolated by using wells (e.g., junction isolation with guard rings), in the case of bulk substrates (e.g., silicon), the bulk substrate will generally still remain coupled to the sensitive circuitry.